The invention relates to orthopedic (bone) implants which are used to replace a missing or diseased portion of bone and, more specifically, to apparatus and methods for manufacturing such implants. Several conditions can lead to the loss of bone including trauma, arthritic diseases, tumors, musculoskeletal defects, and the replacement of a failed implant.
An intramedullary implant is generally used in long bones (i.e., the femur and humerus), and is inserted into the medullary canal, which runs through the diaphysis (shaft) of the bone and is filled with bone marrow. A long bone implant is one of two different types of intramedullary implants, the other being categorized as joint replacements. The joint replacement implants (i.e., a hip or knee implant) have a much more complicated geometry, than the rod-like, long bone replacement. Both types of implants have shown similar modes of failure in clinical studies.
The intramedullary implants being used today are generally fabricated from metal, using an alloy of either titanium (Ti) or cobalt chrome (Co--Cr). The joint replacement implants are primarily made with a Co--Cr alloy containing molybdenum (Mo), which is added to improve the wear resistance properties of the material, an important consideration when the implant is used to replace articulating surfaces.
Long bone replacement implants are most commonly fabricated from Ti, either in its commercially pure state or as an alloy with aluminum and vanadium. These materials have been experimentally and clinically proven to be biocompatible. It is not completely understood biochemically, but the bone tissue grows and attaches to the surface of Ti more readily than to other materials. This property allows Ti to aid in the fixation of the implant to bone, an extremely important factor directly affecting the implant's duration of success.
It is also important for implant success that the implant remain stationary so the bone tissue can begin to grow around it. Initial stabilization is achieved through the use of bone cement applied during surgery, which acts as a filler between the bone and the implant. The interfacial space is filled with cement to stabilize the implant and inhibit motion. The bone cement material is a thermoset particulate composite polymer called polymethylmethacrylate (PMMA).
Long term stabilization of the implant in bone is achieved by having a porous coating on the surface of the implant. The porous coating is either added or molded onto the surface of the implant. Ti or hydroxyapatite (HA) are two materials with good biocompatibility and/or biostimulating factors commonly used to create this porous coating. Ti is sintered onto the surface of the metal (e.g., Ti) implant in either a mesh of crimped wire or a random array of particulates. The HA is applied to the surface of the implant using plasma spraying techniques.
The porous coating must have large enough pores to allow the bone cells to travel through and create a strong interlocking fixation by reconnecting with adjacent bone tissue throughout the mesh. This method of fixation relies on the connection of the bone tissue to hold the implant in place. If the bone tissue does not grow fast or is not strong enough, the implant is not completely stabilized and micromotion can occur.
Problems with current implant designs originate from the difference in mechanical properties between the materials used in the implant system and the bone itself. The Ti alloy has an elastic modulus equal to 110.3 GPa (16.0.times.10.sup.6 psi), and the Co--Cr alloy has an elastic modulus equal to 210.3 GPa (30.5.times.10.sup.6 psi). In comparison to the modulus of cortical bone, equal to about 13.8 GPa (2.0.times.10.sup.6 psi), these metallic implants are a minimum of eight times stiffer. This large gradient causes stress shielding across the implant-bone interface, where the implant supports and absorbs most of the load and leaves the bone virtually inactive and unstressed.
As stated in Wolff's law, bone needs to be cyclically stressed to survive and remain strong enough to support the body. The shielded, unstressed bone around a metal implant begins to resorb and cavities form between the implant and the bone. The cavities weaken the fixation and allow micromotion of the implant in the bone, eventually producing local wear debris. Microscopic foreign body wear debris in the surrounding tissue will trigger the body's defense mechanism and cause infectious reactions. Loosening of the implant is irreversible without intervention and ultimately leads to a revision operation. A patient can only undergo two or three additional procedures before the bone becomes too weak and osteoporotic to support another replacement and is considered non-functional.
A polymer bone implant minimizes, if not eliminates, the stress shielding effect created by a metal implant, thus, leading to a longer implant lifetime in the body. The polymer bone implant can comprise a thermoplastic polymer with an elastic modulus approximating the modulus of bone or a composite comprising a thermoplastic polymer and a reinforcing material, the composite also having an elastic modulus approximating the elastic modulus of bone.
The final step in the manufacture of the polymer bone implant is the application or formation of the porous coating on the surface of the implant to create the porous environment for bone ingrowth, as discussed above. The coating can comprise hydroxyapatite applied to the surface, a roughness formed on the surface, or a biocompatible material such as titanium. The use of this latter type of porous coating requires new apparatus and methods for embedding the biocompatible material in the polymer bone implant surface.